glTF: Fix override materials and non-empty arrays

Keep track of MeshInstance and GeometryInstance override materials in the GLTFMesh object.
Ensure all arrays are non-empty to conform with "minItems":1 in glTF spec.

modules/gltf/doc_classes/GLTFMesh.xml

@@ -9,6 +9,8 @@
 	<members>
 		<member name="blend_weights" type="PackedFloat32Array" setter="set_blend_weights" getter="get_blend_weights" default="PackedFloat32Array()">
 		</member>
+		<member name="instance_materials" type="Array" setter="set_instance_materials" getter="get_instance_materials" default="[]">
+		</member>
 		<member name="mesh" type="ImporterMesh" setter="set_mesh" getter="get_mesh">
 		</member>
 	</members>

modules/gltf/gltf_document.cpp

@@ -795,6 +795,9 @@ Error GLTFDocument::_encode_buffer_views(Ref<GLTFState> state) {
 		buffers.push_back(d);
 	}
 	print_verbose("glTF: Total buffer views: " + itos(state->buffer_views.size()));
+	if (!buffers.size()) {
+		return OK;
+	}
 	state->json["bufferViews"] = buffers;
 	return OK;
 }
@@ -884,6 +887,9 @@ Error GLTFDocument::_encode_accessors(Ref<GLTFState> state) {
 		accessors.push_back(d);
 	}
 
+	if (!accessors.size()) {
+		return OK;
+	}
 	state->json["accessors"] = accessors;
 	ERR_FAIL_COND_V(!state->json.has("accessors"), ERR_FILE_CORRUPT);
 	print_verbose("glTF: Total accessors: " + itos(state->accessors.size()));
@@ -2112,6 +2118,7 @@ Error GLTFDocument::_serialize_meshes(Ref<GLTFState> state) {
 		if (import_mesh.is_null()) {
 			continue;
 		}
+		Array instance_materials = state->meshes.write[gltf_mesh_i]->get_instance_materials();
 		Array primitives;
 		Dictionary gltf_mesh;
 		Array target_names;
@@ -2431,7 +2438,14 @@ Error GLTFDocument::_serialize_meshes(Ref<GLTFState> state) {
 				}
 			}
 
-			Ref<BaseMaterial3D> mat = import_mesh->get_surface_material(surface_i);
+			Variant v;
+			if (surface_i < instance_materials.size()) {
+				v = instance_materials.get(surface_i);
+			}
+			Ref<BaseMaterial3D> mat = v;
+			if (!mat.is_valid()) {
+				mat = import_mesh->get_surface_material(surface_i);
+			}
 			if (mat.is_valid()) {
 				Map<Ref<BaseMaterial3D>, GLTFMaterialIndex>::Element *material_cache_i = state->material_cache.find(mat);
 				if (material_cache_i && material_cache_i->get() != -1) {
@@ -2475,6 +2489,9 @@ Error GLTFDocument::_serialize_meshes(Ref<GLTFState> state) {
 		meshes.push_back(gltf_mesh);
 	}
 
+	if (!meshes.size()) {
+		return OK;
+	}
 	state->json["meshes"] = meshes;
 	print_verbose("glTF: Total meshes: " + itos(meshes.size()));
 
@@ -3454,6 +3471,9 @@ Error GLTFDocument::_serialize_materials(Ref<GLTFState> state) {
 		}
 		materials.push_back(d);
 	}
+	if (!materials.size()) {
+		return OK;
+	}
 	state->json["materials"] = materials;
 	print_verbose("Total materials: " + itos(state->materials.size()));
 
@@ -4837,6 +4857,9 @@ Error GLTFDocument::_serialize_animations(Ref<GLTFState> state) {
 		}
 	}
 
+	if (!animations.size()) {
+		return OK;
+	}
 	state->json["animations"] = animations;
 
 	print_verbose("glTF: Total animations '" + itos(state->animations.size()) + "'.");
@@ -5057,6 +5080,18 @@ GLTFMeshIndex GLTFDocument::_convert_mesh_to_gltf(Ref<GLTFState> state, MeshInst
 	}
 	Ref<GLTFMesh> gltf_mesh;
 	gltf_mesh.instantiate();
+	Array instance_materials;
+	for (int32_t surface_i = 0; surface_i < current_mesh->get_surface_count(); surface_i++) {
+		Ref<Material> mat = current_mesh->get_surface_material(surface_i);
+		if (p_mesh_instance->get_surface_override_material(surface_i).is_valid()) {
+			mat = p_mesh_instance->get_surface_override_material(surface_i);
+		}
+		if (p_mesh_instance->get_material_override().is_valid()) {
+			mat = p_mesh_instance->get_material_override();
+		}
+		instance_materials.append(mat);
+	}
+	gltf_mesh->set_instance_materials(instance_materials);
 	gltf_mesh->set_mesh(current_mesh);
 	gltf_mesh->set_blend_weights(blend_weights);
 	GLTFMeshIndex mesh_i = state->meshes.size();

modules/gltf/gltf_mesh.cpp

@@ -36,9 +36,12 @@ void GLTFMesh::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_mesh", "mesh"), &GLTFMesh::set_mesh);
 	ClassDB::bind_method(D_METHOD("get_blend_weights"), &GLTFMesh::get_blend_weights);
 	ClassDB::bind_method(D_METHOD("set_blend_weights", "blend_weights"), &GLTFMesh::set_blend_weights);
+	ClassDB::bind_method(D_METHOD("get_instance_materials"), &GLTFMesh::get_instance_materials);
+	ClassDB::bind_method(D_METHOD("set_instance_materials", "instance_materials"), &GLTFMesh::set_instance_materials);
 
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "mesh"), "set_mesh", "get_mesh");
 	ADD_PROPERTY(PropertyInfo(Variant::PACKED_FLOAT32_ARRAY, "blend_weights"), "set_blend_weights", "get_blend_weights"); // Vector<float>
+	ADD_PROPERTY(PropertyInfo(Variant::ARRAY, "instance_materials"), "set_instance_materials", "get_instance_materials");
 }
 
 Ref<ImporterMesh> GLTFMesh::get_mesh() {
@@ -49,6 +52,14 @@ void GLTFMesh::set_mesh(Ref<ImporterMesh> p_mesh) {
 	mesh = p_mesh;
 }
 
+Array GLTFMesh::get_instance_materials() {
+	return instance_materials;
+}
+
+void GLTFMesh::set_instance_materials(Array p_instance_materials) {
+	instance_materials = p_instance_materials;
+}
+
 Vector<float> GLTFMesh::get_blend_weights() {
 	return blend_weights;
 }

modules/gltf/gltf_mesh.h

@@ -43,6 +43,7 @@ class GLTFMesh : public Resource {
 private:
 	Ref<ImporterMesh> mesh;
 	Vector<float> blend_weights;
+	Array instance_materials;
 
 protected:
 	static void _bind_methods();
@@ -52,5 +53,7 @@ public:
 	void set_mesh(Ref<ImporterMesh> p_mesh);
 	Vector<float> get_blend_weights();
 	void set_blend_weights(Vector<float> p_blend_weights);
+	Array get_instance_materials();
+	void set_instance_materials(Array p_instance_materials);
 };
 #endif // GLTF_MESH_H